How does Eco mode really work?

[Don]

You are correct that energy can neither be created or destroyed, so there is a finite amount of kinetic energy in the mass of the car, rolling at a given speed. If you give away 10% or 20% of that to friction braking by hitting the brake pedal, that's energy you're never going to get back into the battery pack - Hitting the brake pedal before 11 mph is a bad thing, energy-wise. All regen stops at 11 mph (18 kph)

Observing the charge meter on the dash alters your perception of that you *think* is going on, because it only displays the first 50 amps of regeneration and we know the car is capable of more than twice that amount . . . . so when you see the gauge pegged, you could be getting anywhere from about 50 amps to 110 amps of regenerated power back into the pack - Both look the same

The Prius is a very different animal because of it's hybrid synergy drive which acts like a continuously variable transmission, so not everything you understand to be true of hybrid cars translates directly to the very simple and efficient direct drive arrangement of a true EV

True, the car only has ONE regenerative braking system (and it's much simpler than the one used in the Prius) but like most things on this car, it's computer controlled, so they can make it have as many different modes as they want

I agree with you that using the B mode could be a bad thing if it's not done properly. A goodly percentage of people who grew up solely driving automatic transmissions unconsciously alternate between gently pressing the accelerator until they notice they are going a few mph too fast and then gently easing up on the pedal until they notice they're going a few mph too slow . . . . and then repeat every few hundred yards until they get where they're going. If this is how you drive, the B mode will most certainly waste some energy for you - You need a car which basically disconnects the engine from the wheels when you lift your foot, like auto trans ICE's do. The iMiEV is much more like a manual trans car in this regard, because the 'engine' is always connected to the wheels, so lifting your foot is always a waste of energy, even if you do it in Neutral, because the wheels are then using some energy turning over the entire unpowered drivetrain

The longer you roll (time-wise) the more kinetic energy is lost due to wind resistance and rolling resistance, so it just seems like common sense to me that to get the most regen possible, you need to get it as quickly as possible, so that you're not wasting as much of it on wind and friction

If Joe's ammeter could take cumulative measurements of the amps regenerated from the time the pedal is lifted at 60 mph until the regen kicks out at 11 mph, we could get some actual numbers of power generated in the B, Eco and D modes. B mode would certainly provide the most because it's not wasting as much energy on wind and rolling resistance

Don

 

[fjpod]

Also IMHO, I think B mode is bad, unless you are doing a lot of downhill where you need to control your speed, because if you ease up on the go pedal just a little bit due to fatigue or distraction, the car slows down a lot...and then you have to use more energy to get back up to where you were.

I think Eco mode is bad because it requires the use of the brake pedal more frequently than does B mode. If you press down on the brake pedal just a little bit too much due to fatigue or distraction, the friction brakes are applied which wastes energy.

Not likely unless you panic stop. Just watch what happens on the ampmeter as you gradually bear down on the brake pedal. If you were engaging friction braking, the amount of regen would not be going up. Of course as you come close to stopping, you start to lose regen but not because of friction braking, just because you are not going fast anymore and so you get less regen.

I rarely drive my imiev at speeds where wind drag is a factor.

 

[Don]

Also IMHO, I think B mode is bad, unless you are doing a lot of downhill where you need to control your speed, because if you ease up on the go pedal just a little bit due to fatigue or distraction, the car slows down a lot...and then you have to use more energy to get back up to where you were.

I think Eco mode is bad because it requires the use of the brake pedal more frequently than does B mode. If you press down on the brake pedal just a little bit too much due to fatigue or distraction, the friction brakes are applied which wastes energy.

Not likely unless you panic stop. Just watch what happens on the ampmeter as you gradually bear down on the brake pedal. If you were engaging friction braking, the amount of regen would not be going up.

Yes, it would - You can even see the amp meter pegged all the way to the left while you're friction braking. Both friction braking on the front wheels and maximum regenerative braking on the rear wheels can (and does) happen simultaneously. Whenever the pedal is lightly pressed, maximum regen is activated and as you press a little harder to activate the friction brakes, the regen will still be pegging the meter until your speed lowers to the point where 50 amps of regen is no longer possible

Don

 

[fjpod]

Are you sure the regen braking blends with the friction?

 

[Don]

The Wiki describes The iMiEV's brakes as "conventional power assisted hydraulic brakes"

Here's a reviewer who seems to be familiar with how things work on both our car and hybrids and the difference between them

"The transmission has the familiar PRND settings, plus Eco and B modes. Drive is the most like a normal car in that it lets you coast along when you lift off the accelerator. Eco gives you more engine braking (or motor/generator braking to be exact). The B setting, which we've seen on some hybrids, gives you even more engine braking. Coasting down a hill, I switched from Drive to Eco to B and felt the car decelerate faster with each change. With the greater braking comes more battery regeneration, as indicated on the charge gauge. This type of braking will boost the electric vehicle’s range.
With its increased regeneration, B turned out to be the most efficient mode. By controlling the car's acceleration and deceleration with the accelerator pedal, I maximized regeneration and made very little use of the brakes. Unlike hybrids, the i-MiEV's brakes don't regenerate energy. They're just normal brakes, and as such, they operate and feel better than brakes on most hybrid cars. But they actually rob you of efficiency by turning your inertia into heat in the brake pads rather than electricity in the generator. The best-case scenario is that you use them as little as possible, and the B setting does this best."

I've driven in the B mode exclusively for 11,000 miles and many times I get 70 or 75 miles or range while running the A/C so I'm pretty sure the way I use it to avoid friction braking isn't costing me any range . . . . and Mitsu says use of regen in city driving can extend range up to 30% - I'm pretty sure they're not talking about using D mode when they make that claim

Don

 

[tonymil]

Here's why I think Don is right about more regen in B mode than in D mode. Gravity is a constant and continues to work against motion over time. The longer the time period, the greater slowing effect gravity has. When B mode is used it takes less time to get down to 10 mph (when the car starts to coast) than it would in D mode. Less time means you lessen the effect of gravity which means that more of the car's slowing was caused by the car's energy recapture. Of course the key to maximizing regen isn't just which mode you use but also when you engage regen. If you engage it too soon you may have to step on the accelerator to get the car to the appropriate stopping point on the road. If you engage it too late you will have to press the brakes early which means regen is lost to the friction brakes. Then of course there's the reaction of the driver behind you. using regen, especially B mode, means the car slows down a lot quicker than most of the other cars on the road. It's amazing how impatient cars can be to get to a red light, some of them hate crawling to the light and want to get there quickly. I often have to "tap" the brake to allow the brake lights to "flick" on and off to get the driver behind me to slow down.

 

[PV1]

1. Activates an electro-mechanical constraint on the accelerator, making it physically harder to depress the pedal. This is simply a tactile feedback device meant to counteract a driver's tendency to leadfoot.

2. Changes the gain of the controller's motor drive in response to the accelerator pedal position. Specifically, for a given pedal depressed position, you get more power in either D or B modes than you do in Eco Mode. My impression is that this is not linear, because at the upper end if you do indeed "floor it" in Eco, you WILL get full power (a good safety feature).

3. Increases the amount of regeneration over what you get in D mode, but less than what you get in B mode. What's nice about this is that when you want to slow down you can "downshift" through the modes and modulate you deceleration without ever tapping the brakes.

In the first two cases, increased efficiency (and thus improved range) comes from reducing peak power consumption, but resulting in simply getting up to speed slower than a leadfoot.

HTH

I don't agree with item 1. The pedal has the same resistance in all modes. ECO seems to have more resistance because you're pushing much further down on the pedal compared to D or B, but it doesn't actually change (does the pedal push up or drop down when switching between D and ECO?)

Item 2, the pedal is dampened until 2/3 power, at which point it gets super-sensitive to full power.

 

[fjpod]

The Wiki describes The iMiEV's brakes as "conventional power assisted hydraulic brakes"

Here's a reviewer who seems to be familiar with how things work on both our car and hybrids and the difference between them

"The transmission has the familiar PRND settings, plus Eco and B modes. Drive is the most like a normal car in that it lets you coast along when you lift off the accelerator. Eco gives you more engine braking (or motor/generator braking to be exact). The B setting, which we've seen on some hybrids, gives you even more engine braking. Coasting down a hill, I switched from Drive to Eco to B and felt the car decelerate faster with each change. With the greater braking comes more battery regeneration, as indicated on the charge gauge. This type of braking will boost the electric vehicle’s range.
With its increased regeneration, B turned out to be the most efficient mode. By controlling the car's acceleration and deceleration with the accelerator pedal, I maximized regeneration and made very little use of the brakes. Unlike hybrids, the i-MiEV's brakes don't regenerate energy. They're just normal brakes, and as such, they operate and feel better than brakes on most hybrid cars. But they actually rob you of efficiency by turning your inertia into heat in the brake pads rather than electricity in the generator. The best-case scenario is that you use them as little as possible, and the B setting does this best."

Well this reviewer is wrong. The American version of the iMiev does have regenerative brakes. Typically the friction brakes engage all of a sudden if a certain amount of pedal depression is sensed with a certain amount of forward inertia. They typically do not mix and blend with simultaneous regenerative braking. Again, I am not an engineer with MM, but this is my understanding of regenerative braking.

 

[RobbW]

Yeah, that Wiki article doesn't make any sense. Where does the author get off saying the i-MiEV doesn't have regenerative braking? I thought that was the whole point! All of MM's literature on the i-MiEV states regenerative braking system.

 

[fjpod]

Here's why I think Don is right about more regen in B mode than in D mode. Gravity is a constant and continues to work against motion over time. The longer the time period, the greater slowing effect gravity has. When B mode is used it takes less time to get down to 10 mph (when the car starts to coast) than it would in D mode. Less time means you lessen the effect of gravity which means that more of the car's slowing was caused by the car's energy recapture. Of course the key to maximizing regen isn't just which mode you use but also when you engage regen. If you engage it too soon you may have to step on the accelerator to get the car to the appropriate stopping point on the road. If you engage it too late you will have to press the brakes early which means regen is lost to the friction brakes. Then of course there's the reaction of the driver behind you. using regen, especially B mode, means the car slows down a lot quicker than most of the other cars on the road. It's amazing how impatient cars can be to get to a red light, some of them hate crawling to the light and want to get there quickly. I often have to "tap" the brake to allow the brake lights to "flick" on and off to get the driver behind me to slow down.

Sorry Tony, I think the effects of gravity on the regenerative braking equations compared to the kinetic forward motion of the car, is zilch...unless you are stopping the car for hours on end, in which place you would be going nowhere anyway.

...just citing the physics as I see it.

 

[Don]

The Wiki describes The iMiEV's brakes as "conventional power assisted hydraulic brakes"

Here's a reviewer who seems to be familiar with how things work on both our car and hybrids and the difference between them

"The transmission has the familiar PRND settings, plus Eco and B modes. Drive is the most like a normal car in that it lets you coast along when you lift off the accelerator. Eco gives you more engine braking (or motor/generator braking to be exact). The B setting, which we've seen on some hybrids, gives you even more engine braking. Coasting down a hill, I switched from Drive to Eco to B and felt the car decelerate faster with each change. With the greater braking comes more battery regeneration, as indicated on the charge gauge. This type of braking will boost the electric vehicle’s range.
With its increased regeneration, B turned out to be the most efficient mode. By controlling the car's acceleration and deceleration with the accelerator pedal, I maximized regeneration and made very little use of the brakes. Unlike hybrids, the i-MiEV's brakes don't regenerate energy. They're just normal brakes, and as such, they operate and feel better than brakes on most hybrid cars. But they actually rob you of efficiency by turning your inertia into heat in the brake pads rather than electricity in the generator. The best-case scenario is that you use them as little as possible, and the B setting does this best."

Well this reviewer is wrong. The American version of the iMiev does have regenerative brakes. Typically the friction brakes engage all of a sudden if a certain amount of pedal depression is sensed with a certain amount of forward inertia. They typically do not mix and blend with simultaneous regenerative braking. Again, I am not an engineer with MM, but this is my understanding of regenerative braking.

I think you're probably correct in your understanding of regen . . . . if you're driving a Prius

He didn't say we don't have regenerative braking - In fact, he accurately described how it works in the various driving modes. What he's talking about is our simple, conventional hydraulic brakes, which work just like they do on any conventional car. We do not combine friction and regen on the front axle like Prius's do. We have regen only available at the rear and friction only at the front. They don't 'mix' . . . . like they do on a Prius. Whenever the pedal is depressed by any amount, we have max regen on the rear and when the pedal is depressed more than just a little, we have friction only on the front axle and drum brake friction aided by regen on the rear. A very simple system compared with the way the Prius does it

Don

 

[Vike]

Been following all this with some interest, but just weighing in on two points:

1) Based on the excerpt posted here, I would have to say that the reviewer Don cites does not understand that depressing the brake pedal causes the motor to go into maximum regen. Perhaps Don knows better because of something written elsewhere in the wiki. But thanks for the rest of this - I found the discussion of how the car combines friction and regen interesting, I'll pay more attention to this as I drive.

2) I'm not sure why there's so much back and forth about the amount of regen in the various modes. You get more total regen from B than E, more from E than D; perhaps this is one of those cases where one can overthink oneself. I'm not sure why fjpod thinks that losses to rolling resistance are negligible, but they certainly are not. Get onto a level empty road somewhere, put the car in N, and note how it eventually but very definitely comes to a stop all by itself. All those losses need to be deducted from the regen in whatever mode you're in, based on the amount of time it takes you to decelerate, so the higher the rate of regen, the more total energy you'll recapture (as opposed to being taken up in all those other losses).

On the other hand, sure, air resistance declines rapidly with speed . . . and I have no idea what "gravity" was supposed to mean in this context, unless perhaps in confusion with rolling resistance.

 

[NeilBlanchard]

The purpose of the car is to move you from A to B and if you have to slow it down a lot, you're accelerating too much. Too much acceleration already wastes energy, and then having to use regen to regain *some* of what you threw away - doesn't help!

 

[RobbW]

The purpose of the car is to move you from A to B and if you have to slow it down a lot, you're accelerating too much. Too much acceleration already wastes energy, and then having to use regen to regain *some* of what you threw away - doesn't help!

This is very true, Neil. Unfortunately, here in the Chicagoland area, practicing maximum driving efficiency will most likely get you shot, especially during the rush-hour commute! I'm forced to adapt my driving style to fit local traffic conditions. Luckily, I'm driving all electric, which means I'm still saving fuel money and reducing my environmental impact regardless of how I have to drive in suburban traffic!

 

[HParkEV]

... They don't 'mix' . . . . like they do on a Prius. Whenever the pedal is depressed by any amount, we have max regen on the rear and when the pedal is depressed more than just a little, we have friction only on the front axle and drum brake friction aided by regen on the rear. A very simple system compared with the way the Prius does it

Don

I don't agree with this statement - that basically says the regen brakes are non-variable.
I'm pretty convinced that while you are braking gently, if you increase the pressure on the brake pedal, you're also increasing regen. It's pretty easy to see when going down a slight hill in D mode, but not too fast, you need the regen gauge only part way into the blue area with light pressure on the brake. If you apply more pressure on the brake, you will see the gauge go deeper into the blue area, so it seems to me that we are getting 'mixing' with both more regen and more friction braking at the same time, as the brake pressure increases.
This is much harder to notice in Eco or B modes because we're maxing out the regen amp gauge at lower speeds and much lighter brake pressures. Because of this, I would speculate that with a given brake pressure that is higher than just activating the brake lights, we will get more total energy back from regen in B mode or Eco mode than in D mode. D mode seems to be easier on the regen no matter how much pressure is on the brake pedal, probably up to some limit where you reach max regen no matter which mode you are in.
This is just speculation on my part, based on observation of the amp needle while braking.

 

[fjpod]

I seriously doubt MM "limits" the amount of regen you get in any drive mode (other than what is the maximum capacity for the system). I would be inefficient to throw away say, half of the regen in D, just to make it seem lighter. It's just that you are getting the regen at a slower rate. It's all based on time.

 

[Don]

... They don't 'mix' . . . . like they do on a Prius. Whenever the pedal is depressed by any amount, we have max regen on the rear and when the pedal is depressed more than just a little, we have friction only on the front axle and drum brake friction aided by regen on the rear. A very simple system compared with the way the Prius does it

Don

I don't agree with this statement - that basically says the regen brakes are non-variable.

If we're talking about the brake pedal, regen isn't variable

A very light pressure on the brake pedal sets the car for maximum regen. Any more pressure than that and you're friction braking. Other than the fact that any pressure on the pedal initiates regen, the brake system is just the very conventional hydraulic, power assist, ABS system used on most ICE's

Don

 

[Vike]

I seriously doubt MM "limits" the amount of regen you get in any drive mode (other than what is the maximum capacity for the system). I would be inefficient to throw away say, half of the regen in D, just to make it seem lighter. It's just that you are getting the regen at a slower rate. It's all based on time.

Yes, it's all based on time - and that's exactly the point. Nobody's saying that any mode deliberately "throws away" or "limits" regeneration. But time matters, because the longer it takes to recapture/convert momentum to electrical power, the more of that power is lost to other factors such as air resistance, rolling resistance, bearing friction, etc. Let's say that you take your foot off the juice, at which point regen kicks in, and continues until vehicle speed falls to 11 mph. If "B" takes 5 seconds to get you there, and "D" takes 15 seconds, that's 10 extra seconds for all those other losses to "steal" power from the vehicle's momentum before regen gets to recapture it. You'd need a serious engineer to tell us what all that adds up to, and maybe it's not all that much, but I hope it's obvious that it's not zero.

Okay - seeing that the horse stopped drawing breath a few lashes ago, I promise I have nothing more to say on this specific point. :roll:

 

[Vike]

I don't agree with this statement - that basically says the regen brakes are non-variable.

If we're talking about the brake pedal, regen isn't variable

A very light pressure on the brake pedal sets the car for maximum regen. Any more pressure than that and you're friction braking.

I'm sorry, but what was the source of your information indicating that regen on braking was "all or nothing"? Because I can't agree, for a couple of reasons.

One is empirical - I tested this this morning on a stretch of level, little used road. From 35 mph while in "D", very light pressure on the brake pedal did indeed yield more regen than I saw from just "coasting" (which still does some light regen in "D"), but not a lot, and definitely less than I saw lifting off the go pedal while in "B" in another trial. More important, as I increased pressure on the pedal, I could see the regen briefly increasing on the power meter (before it got too slow for regen). If the regen behaves as you described, and the increased deceleration was caused only by friction, then the regen could not have increased.

My second objection is partly empirical, and partly a (theoretical?) safety concern. Maximum regen yields quite a bit of stopping power - if the system did behave as you described, the car would have extremely grabby brakes, which it does not. It just wouldn't be safe (or efficient) in stop and go traffic, where I have in fact found the i-MiEV's behavior quite smooth and predictable; it's probably the most "drivable" car I've ever had (a typical joy of the EV experience )

I would invite anyone to drive the same route in "D" and then again in "B", and see if you can't stop more gradually braking from "D" than you can yanking your foot off the go pedal in "B". I don't think you'll find it difficult. Since we know "B" only does regen and does NOT automatically apply friction braking, then if braking regen worked as Don described, ANY braking from "D" would have to be AT LEAST as abrupt as "coasting down" in "B" - and I don't think it is. But again, try it for yourself (and safety first!).

I do understand that friction brakes come into play as more brake pressure is applied, absolutely. But while I haven't tinkered with this a lot, so don't have a sense as to just how far it goes, based on the above I'm fairly certain that the initial regen component of our braking is variable and not a "toggle".

 

[fjpod]

Wait, I think the horse batted an eyelash...

I doubt that..., no, I would assert that it takes more than just a "little more pressure" on the brake pedal to engage friction braking. One can determine this by just watching the ampmeter and playing with the brake pedal a little while going down a steep hill.

Ask an electrical engineer if it is better to charge a battery slowly over a longer period of time, or quickly over a shorter period. What do you think he/whe will say? The battery will take the charge better over a longer period. Same principle applies here as when comparing DC fast charging over Level 1 or 2. Anything you may think you gain from outsmarting gravity or friction is negated by the beneficial effects of slower charging.

Again, I don't think MM dummied down the regen with the brakes or D mode to give us less recapture efficiency. The brake pedal allows us to regulate the amount of regen we get over a certain period of time, just like the driving modes do. There is only one regen system. The regen you get from the brakes is the same system that gives you the regen from the drive modes. Pick your poison, or pick your convenience.